1. Field of the Invention
The present invention relates to a surface-mount type quartz crystal oscillator, and more particularly, to a surface-mount type crystal oscillator having an inspection terminal for inspecting a vibration characteristic of a quartz crystal blank.
2. Description of the Related Art
Crystal oscillators accommodating a quartz crystal blank and an IC (integrated circuit) chip provided with an oscillation circuit using this crystal blank in a container are used as frequency and time reference sources for various devices. Especially, a surface-mount type crystal oscillator using a container for surface mounting is small and light, and is therefore generally incorporated particularly in portable electronic devices. One of such crystal oscillators is a surface-mount type crystal oscillator for clocks having a frequency-temperature characteristic dependent on a temperature characteristic of a crystal blank.
FIG. 1A is a partially cutaway front view showing an example of a conventional surface-mount type crystal oscillator, and FIG. 1B is a plan view of the crystal oscillator shown in FIG. 1A with its metal cover, crystal blank and third frame wall layer removed.
The crystal oscillator shown in FIG. 1 accommodates IC chip 2 and crystal blank 3 in container body 1 in which a recess is formed. Metal cover 4 covers the top surface of container body 1 surrounding the opening of the recess, and IC chip 2 and crystal blank 3 are hermetically encapsulated within the recess. Container body 1 has a substantially rectangular plane shape and the recess is provided on one principal surface thereof. Such container body 1 is made of laminated ceramics including substantially rectangular, flat bottom wall layer 1a and frame wall 1b provided on one principal surface of bottom wall layer 1a and having an aperture. The recess is formed of the aperture of frame wall 1b. Here, bottom wall layer 1 a is made of a single-layer ceramic sheet, but frame wall 1b is made of three layers of ceramic sheets; first layer 1b1, second layer 1b2 and third layer 1b3 from the bottom wall layer side. Since the aperture provided in third layer 1b3 is made larger than the apertures provided in first layer 1b1 and second layer 1b2, a step portion is formed in the inner wall of the recess. A pair of holding terminals 5 are provided on this step portion. A plurality of circuit terminals are provided on the inner bottom surface of the recess and two of these circuit terminals are electrically connected to holding terminals 5 via a conductive path (not shown) formed in container body 1.
In four corners of the outer bottom surface of container body 1, that is, the other principal surface of bottom wall layer 1a, there are provided mounting terminals 6 electrically connected to the circuit terminals and used to surface-mount this crystal oscillator on a wiring board. Mounting terminals 6 include a power supply terminal, an output terminal, a grounding terminal and a standby terminal. On the outer side surfaces of container body 1, there are provided inspection terminals 7a, 7b. Inspection terminals 7a, 7b are used to measure a vibration characteristic of crystal blank 3 from outside and are electrically connected to a pair of circuit terminals which are connected to holding terminals 5 Inspection terminals 7a, 7b are formed on a pair of opposite outer side surfaces of container body 1, on the sides of frame wall 1b except the highest layer, that is, third layer 1b3. At the positions at which inspection terminals 7a, 7b are formed, as shown in FIG. 1B, substantially rectangular depressed portions 8 are formed on the outer sides of frame wall 1b and inspection terminals 7a, 7b are formed on the inner surfaces of depressed portions 8.
As shown in FIG. 2, crystal blank 3 is, for example, a substantially rectangular AT-cut quartz crystal blank and excitation electrodes 10a are provided on both principal surfaces thereof. Lead-out electrodes 10b extend from excitation electrodes 10a toward both sides of one end of crystal blank 3, respectively. Both sides of one end of crystal blank 3 toward which lead-out electrodes 10b extend are fixed to holding terminals 5 using conductive adhesive 11 or the like, and crystal blank 3 is thereby electrically and mechanically connected to holding terminals 5 and held in the recess of container body 1.
IC chip 2 has a substantially rectangular shape and is made up of electronic circuits including at least an oscillation circuit using crystal blank 3, the electronic circuits integrated on a semiconductor substrate. In IC chip 2, since the electronic circuits such as an oscillation circuit are formed on one principal surface of the semiconductor substrate through a normal semiconductor device fabrication process, the principal surface on which these electronic circuits are formed among the pair of the principal surfaces of the semiconductor substrate will be referred to as a “circuit forming surface” of the IC chip. A plurality of IC terminals for connecting IC chip 2 to an external circuit are also formed on the circuit forming surface. IC terminals include a power supply terminal, grounding terminal, oscillation output terminal, standby terminal, a pair of connection terminals for connection with crystal blank 3, or the like. IC chip 2 is electrically and mechanically connected to container body 1 and also electrically connected to crystal blank 3 and mounting terminals 6 by fixing the IC terminals to the circuit terminals formed on the inner bottom surface of the recess through ultrasonic thermocompression bonding using, for example, bumps 9.
A metal ring is formed on the top surface of third layer 1b3 so as to surround the recess of container body 1 and metal cover is bonded to the metal ring through seam welding or the like. The recess is hermetically closed in this way.
Container body 1 of the above described crystal oscillator is formed by stacking and burning unburned ceramic raw sheets, that is, ceramic green sheets, corresponding to bottom wall layer 1a and first to third layers 1b1 to 1b3. In this case, by stacking and burning together the ceramic green sheets corresponding in size to a plurality of container bodies 1 and then dividing the burned body into individual container bodies 1, a plurality of container bodies 1 can be obtained simultaneously through single burning. Before stacking and then burning the ceramic green sheets, inspection terminals 7a, 7b and depressed portions 8 are formed. Holding terminals 5, mounting terminals 6, inspection terminals 7a, 7b and circuit terminals are formed as electrode layers provided on the surface of the laminated ceramics.
FIG. 3 shows manufacturing steps of depressed portions 8 and inspection terminals 7a, 7b. In this figure, parting lines when dividing the burned body into the individual burned container bodies 1 are shown as lines A-A and lines B-B. Here, parting lines A-A are parting lines along the long side of container body 1 and parting lines B-B are parting lines along the short side of container body 1.
Slit-like through holes 8B to be transformed into depressed portions 8 are formed in frame wall sheet 1B12, which is made up of ceramic green sheets 1B1, 1B2 corresponding to first layer 1b1 and second layer 1b2 of frame wall 1b stacked together, on parting lines A-A along the long sides of container bodies 1. When a circuit pattern making up circuit terminals and conductive paths connected to the circuit terminals is formed with tungsten (W) or molybdenum (Mo) using a printing method, tungsten or molybdenum material is charged into through holes 8B and base electrodes are formed on inner surfaces of through holes 8B.
Next, frame wall sheet 1B12, a ceramic sheet corresponding to bottom wall layer 1a and a ceramic sheet corresponding to third layer 1b3 are stacked together to form a container body sheet and these are integrally burned. After burning, nickel (Ni) and gold (Au) films are formed one by one on the base electrode through, for example, electrolytic plating. In the figure, the electrode pattern used in conducting electrolytic plating is omitted for convenience' shake. In this way, inspection terminals 7a, 7b are formed together with holding terminals 5, mounting terminals 6, circuit terminals, and conductive paths connecting these terminals. After this, the container body sheet is divided along parting lines A-A and parting lines B-B to obtain individual container bodies 1.
Next, IC chip 2 and crystal blank 3 are accommodated in the recess of container body 1 obtained in this way and metal cover 4 is bonded to the metal ring surrounding the recess, and this completes the crystal oscillator. When metal cover 4 is bonded through seam welding, stress may be applied to container body 1 due to a difference in thermal expansion coefficients between container body 1 and metal cover 4, and this may cause the condition in which conductive adhesive 11 holds crystal blank 3 to change. Therefore, after metal cover 4 is bonded, a measuring probe (not shown) is made to contact inspection terminals 7a, 7b and the vibration characteristic of crystal blank 3 is tested with crystal blank 3 hermetically sealed within the recess. When the vibration characteristics such as crystal impedance (CI) and frequency temperature characteristic do not satisfy standard values, the crystal oscillator is discarded as a non-conforming product.
In the above described crystal oscillator, inspection terminals 7a, 7b are formed in neither the lowest ceramic layer, that is, bottom wall layer 1a, nor the top ceramic layer, that is, third layer 1b3, in the container body made of laminated ceramics. This prevents inspection terminals 7a, 7b from being electrically short-circuited with the circuit pattern on the wiring board or with metal cover 4.
The surface-mount type crystal oscillator with inspection terminals provided on the sides of the container body made of laminated ceramics is disclosed, for example, in Japanese Patent Laid-Open No. 2000-77942 (JP-2000-077942A), Japanese Patent Laid-Open No. 2003-163540 (JP-2003-163540A) and Japanese Patent Laid-Open No. 2007-142869 (JP-2007-142869).
However, the aforementioned surface-mount type crystal oscillators are becoming more compact and, for example, crystal oscillators having a plane outside size of 2.0×1.6 mm and a height of 0.8 mm or less are under study. When such a size reduction advances, the areas of inspection terminals 7a, 7b are made smaller and especially the length in the vertical direction, that is, the height of the inspection terminal becomes smaller, which may cause insufficient contact between the measuring probe and the inspection terminals in inspecting the vibration characteristic of crystal blank 3.